The present invention relates to a two-dimensional infrared focal plane array in which a thermal type light detector is employed.
The thermal type light detector absorbs the infrared rays, when the infrared rays are irradiated, to raise the temperature and to detect the temperature change. FIG. 21 is a perspective view illustrating the structure of one pixel of the two-dimensional infrared focal plane array in which the conventional thermal type light detector comprising a bolometer thin film in which a resistance value changes due to the temperature is employed. Referring to the drawing, reference numeral 901 shows a substrate composed of semiconductor such as silicon. Reference numeral 910 shows an infrared ray detector portion (hereinafter referred to as detector portion) provided across a space from the silicon substrate. Reference numeral 911 shows a bolometer thin film formed on the infrared ray detector portion. Reference numerals 921 and 922 show support legs for floating and raising the infrared ray detector portion 910 from the silicon substrate. Reference numerals 931 and 932 show metal wirings for flowing current to the bolometer thin film. Reference numeral 940 shows a switch transistor for turning on or off the current flowing through the metal wirings 931, 932, and the bolometer thin film 911. Reference numeral 950 shows a signal line connected with a metal wiring 932. Reference numeral 960 shows a control clock line for controlling the on or off of the switch transistor. Reference numeral 970 shows a metal reflection film comprising the detecting portion and the optical resonant structure to increase the absorption of the infrared rays with the detector portion 910.
FIG. 22 is a sectional view illustrating the sectional structure along the current path of the conventional two-dimensional infrared focal plane array shown in FIG. 21. Referring now to FIG. 22, the same reference numerals are given to the same elements as those shown in FIG. 21 (same throughout the following drawings). Reference numeral 980 shows an insulating film. Reference numeral 990 shows a cavity portion. Reference numerals 930 and 933 show insulating films. Reference numerals 926 and 927 show contact portions. Any switch transistor, signal line, control clock line or the like, not directly related to the invention, has been omitted. A bolometer thin film is formed on the aforementioned detector portion 910. The bolometer thin film is connected with the metal wirings 931 and 932. The bolometer thin film is connected through the contact portions 926 and 927 with a signal readout circuit formed (not shown) on the silicon substrate. The bolometer thin film 911 and the metal wirings 931 and 932 are covered with insulating films 930 and 933 composed of silicon dioxide film (SiO.sub.2) or silicon nitride film (SiN) or the like. The insulating films 930 and 933 form the mechanical structure of the detector portion 910 and the support legs 921 and 922. The insulating film 980 is one for insulating the signal readout circuit formed on the silicon substrate 901, and the metal wirings 931 and 932. On the metal reflection film 970 of the insulating film 980 is positioned a detector portion 910 through the cavity portion 990. On the surface of the metal reflection film 970 is occasionally formed another insulating film.
The operation of the two-dimensional infrared focal plane array using the thermal type light detector will now be described. The infrared rays will be incident from a side where the detector portion 910 exists and will be absorbed by the detector portion 910. Since the incident infrared rays can generate standing waves so that the position of the metal reflection film 970 can be a section due to the existence of the metal reflection film 970, the absorption in the detector portion 910 can be increased by the proper setting of the interval between the detector portion and the metal oxide reflection film. The energies of the infrared rays absorbed by the detector portion 910 are converted into heat to raise the temperature of the detector portion 910. The temperature rise depends upon the amount of the incident infrared rays (the amount of the incident infrared rays depends upon the temperature and the emissivity of the image object). Since the amount of the temperature rise can be obtained by the measuring of the change in the resistance value of the bolometer thin film, the amount of the incident infrared rays irradiated by the image object can be obtained from the change of the resistance value of the bolometer.
When the resistance temperature factor of the bolometer thin film is the same, the resistance change to be obtained by the incident infrared rays of the same amount becomes larger and the sensitivity becomes higher, with larger temperature rise of the detector portion. To increase the temperature rise, it is effective to make any heat escaping to the silicon substrate 901 from the detector portion 910 as small as possible. As a result, the support legs 921 and 922 are designed so that the heat resistance is as large as possible. It is important to make the thermal capacity portion of the detector portion 910 small so that the temperature time constant of the detector portion 910 can be short as compared with the frame time of the image device.
Although the infrared rays become incident onto the area of the whole pixel, only a part are incident onto the detector portion 910 and, thus, contribute to the temperature rise of the detector portion 910 (although some infrared rays incident on the support legs close to the detector portion 910 are effective) and the infrared rays incident onto other parts of the area become ineffective. As the result, to increase the sensitivity, it is effective to make the fill factor larger (a ratio of an area of the detector portion with respect to the pixel area).
In the conventional structure shown in FIG. 21 and FIG. 22, the detector portion 910 has to be formed in an area except for at least the support legs 921, 922 and the contact portions for connecting the readout circuit formed on the support legs and the silicon substrate. The fill factor is subjected to restriction in accordance with the design of the interval tolerance between the support leg and the contact portion, and these portions and the detector portion 910, thus interfering with high sensitivity.
The problem becomes remarkable as the pixels become smaller, thus making it difficult to have higher resolution by using the smaller pixels in such a manner that the sensitivity is retained.
The invention is achieved to resolve the aforementioned problems. An object of the invention is to provide a highly sensitized two-dimensional infrared focal plane array having pixels capable of achieving the higher fill factor independently of the design of the support legs for forming of the adiabatic structure, metal wirings, contact portions and so on, in a two-dimensional infrared focal plane array for forming the thermal type light detector on the substrate the same as that of the signal readout circuit.